Nearly 90% of herbal sports supplements are mislabeled. One in ten contain prohibited substances | The labels of sports supplements are not what they seem to be.

USP just stands for United States Pharmacopeia. The monographs are completely separate from the USP label program. The FDA states that you must meet your product specification, which is what your label claims are derived from.

https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/cfrsearch.cfm?fr=101.36

If you look at the FDA industry guidance, you can see you have to be at 100% or more of the declared value on the label.

https://www.fda.gov/regulatory-information/search-fda-guidance-documents/guidance-industry-guide-developing-and-using-data-bases-nutrition-labeling

In order to evaluate the accuracy of nutrition label information against a standard for compliance purposes, FDA regulations define two nutrient classes (Class I and Class II) (21 CFR 101.9(g)(3)) and list a third group (Third Group) of nutrients (21 CFR 101.9(g)(5)). Class I nutrients are those added in fortified or fabricated foods. These nutrients are vitamins, minerals, protein, dietary fiber, or potassium. Class I nutrients must be present at 100% or more of the value declared on the label ; in other words, the nutrient content identified by the laboratory analysis must be at least equal to the label value. For example, if vitamin C is added in a fortified product and the label states that vitamin C is present at 10% Daily Value (DV), the laboratory value must equal at least 6 mg of vitamin C/serving (i.e., 10% of the 60 mg Reference Daily Intake (RDI) for vitamin C that is specified in 21 CFR 101.9(c)(8)(iv)).

Some people say that you can have 80% of what is claimed on the label, but that only applies to class II nutrients, which are only non-standardized naturally occurring nutrients in other things.

Class II nutrients are vitamins, minerals, protein, total carbohydrate, dietary fiber, other carbohydrate, polyunsaturated and monounsaturated fat, or potassium that occur naturally in a food product. Class II nutrients must be present at 80% or more of the value declared on the label. As an example: If vitamin C is a naturally occurring nutrient in a product, and the product declares 10% DV vitamin C (i.e., 6 mg/serving) on its label, then laboratory analysis must find at least 80% of the label value (80% of 6 mg or 4.8 mg vitamin C/serving) for the product to be in compliance.

So let's say you are selling non-extracted and non-standardized citrus powder. Let's say it normally has 100mg vitamin C. You are allowed a natural variance for that, and can have 80% of that label claim, or 80mg. However, that does not apply to ingredients you specifically put in or fortify at a specific stated concentration. So if you add ascorbic acid to the product, and say it has 100mg, you have to have at least 100mg in the product, or it would be misbranded. Dietary supplement products must have what they claim on the label.